It has long been recognised that rotary cutting machines present considerable safety hazards to both the operator and to bystanders. The cutter blade will often orbit at rotational speeds in excess of 4,500 rpm and objects impacted by the blade can be thrown outwards with considerable energy or the blade may fracture as a result of metal fatigue and itself become a dangerous missile.
There have been a large number of prior proposals seeking to minimise these dangers. But many, such as that of U.S. Pat. No. 3,971,199 for a non-airlift wheel-mounted machine enclose the cutter blade in a manner adversly affecting cutting efficiency. Safety proposals which seriously impair the cutting efficiency of the mower find only limited user acceptance. U.S. Pat. 3,971,199 records that 30% of the 30,000 reports to the United States Public Health Service in 1973 for mower-related injuries were caused by objects thrown by the orbiting blade.
Many strimmers have the facility for replacing a metal cutter blade by a flexible nylon cord which is then used to trim vegetation adjacent walls and other solid surfaces which might damage the metal blade. A rotary cutting machine according to my invention can also have this facility, with the cord mounted instead of the cutter head and blade, or on the cutter head or blade, usually with the guard removed.
Thus I prefer to use a non-flexible cutter blade which may be of metal or a strong plastics material such as nylon which is however pivotally mounted to a cutter head. The histograms of FIG. 1 and FIG. 2 compare the thrown distances for equal weight test specimens which are injected into the path respectively of a propeller-type cutter blade and into the path of a pivotally-mounted cutter blade, the blades being of equal tip path diameter and being driven to orbit at the same speed; these histograms indicate clearly not only that the mean thrown distance is greater with the non-swivel blade but also that the maximum thrown distance is greater. The test conditions were then repeated for the non-swivel and swivel blades using test specimens of different weights, with the distance of the furthest-thrown specimen in each test being taken as the hazard distance (radius) for that specimen weight; the hazard zone (area) whilst being in the ratio of 2.1 to 1 for the respective 12 gram test specimens thrown furthest, increased to 3.0 to 1 for the 31 gram specimens thrown furthest, and to 8.5 to 1 for the 47 gram specimens thrown furthest. Thus pivotally-mounted cutter blades, such as those already known from U.S. Pat. 3,815,234, offer a considerable safety improvement for bystanders; U.S. Pat. 3,815,234 also discloses the provision of an abutment to limit the permitted pivoting movement of the cutter blade relative to the cutter head. Use of a pivotal blade is also advantageous in that it can prolong the life of the drive means since the drive means is not subject to the same degree of reaction impact loading. However the arrangement I propose has significant improvements over that of U.S. Pat. 3,815,234. For instance, my cutter blade is mounted on the cutter head by a bush which is wider than the cutter blade so that the blade can vibrate free in use or be levered free when stationary out of contact with the cutter head, so encouraging pivotal movement even in the presence of grass or dirt between the cutter head and cutter blade. The cutter blade is overlain by a retaining member, such as a washer, which acts to limit movement of the cutter blade away from the plane of the cutter head, I provide retention means to retain the cutter blade should the pivot fracture, and adapted to retain the cutter blade at least until the drive to the cutter head can be stopped or disengaged. The cutter head and the cutter blade retention means can be inter-engaged without first requiring the cutter head to be dismantled. Furthermore the retention means is designed to give adequate warning to the operator of pivot failure. Preferably the retention means is so designed that the cutter blade is planar, having a first opening to locate on a pivot carried by the cutter head, and an arcuate slot normally out of engagement with a retention tongue carried by the cutter head, the tongue being positioned so as to protrude through the slot, the tongue and one wall of the slot forming the said retention means, with an abutment to limit pivoting movement of the cutter blade to prevent engagement of an end of the slot with the tongue so as to prevent premature impact fracture of the tongue and the consequential loss of the retention means, which may not be noticed by the operator. Part of the slot is between the cutter blade pivot and the cutter head axis.